1. Field of the Invention
This invention relates to a reflection layer of a semiconductor light emitter, and more particularly, to a high effective total reflection layer of a semiconductor light emitter which is utilized for a light-emitting diode, a semiconductor laser and so on.
2. Description of the Prior Art
In the semiconductor light emitter, such as the light-emitting diode (LED), the semiconductor laser etc., it is very important to extract light from inside effectively, in other words, to improve an outer radiative efficiency, from a point of obtaining a semiconductor light-emitter of high power and efficiency.
Particularly, in case of using a substrate absorbing a radiative wavelength, it has been devised a countermeasure to prevent the substrate from absorbing the light by a provision of a reflecting layer between the substrate and a light-emitting section, since the light absorption in the substrate may be one of factors to reduce the outer radiative efficiency of the semiconductor light emitter.
As an example of the prior art, FIG. 1, shows a cross-sectional view of an LED having a multi-layer reflection film arranged on an opaque substrate, thereby emitting the light from an upper surface of the LED. This emitter is produced as follows; on the whole surface of an n-type GaAs substrate 510, n-type AlInP/AlGaInP multi-layer reflection layers 511 (layer thickness: 0.041 .mu.m(AlInP); 0.040 .mu.m(AlGaInP), 20 pairs), an n-type AlGaInP cladding layer 512, an undoped AlGaInP emitting layer 513, a p-type AlGaInP cladding layer 514 and a p-type GaAs contact layer 515 are fabricated in order.
Thereafter, a surface electrode 516 is deposited on a surface of the LED and then the electrode 516 and the p-type GaAs contact layer 515 are etched except a central portion of the LED. Further, a back surface electrode 517 is also deposited on a back surface of the LED.
In the above LED, however, since the multi-layer reflection film affords high reflectivity for only light in a specific incident direction, i.e., a vertical incident light in this case, there is caused a problem that, although a beam p traveling straight downward is reflected by the multi-layer reflection film to radiate upward, a beam q traveling downward obliquely is absorbed by the multi-layer reflection film so that it does not contribute to the outward radiation.
Further, since the multi-layer reflection film reflects mainly the light traveling straight downward, the light is radiated through the upper surface of the LED chip, so that the amount of light radiated through the side surfaces of the chip is remarkably small. Such a fact is inconvenient for applying a simple mounting method of the chip described hereinafter.
We now describe a method disclosed in Japanese Patent Application Laid-Open Sho 57 No. 49284 as a simple method to mount the LED on a printed board directly without wire-bonding. As shown in FIG.2A, in the above method, a p-type semiconductor layer 718 is fabricated on an n-type semiconductor substrate 719 to form an electrode 721 on an upper surface of the LED chip, which radiates the light in the vicinity of a pn junction surface 720, and an electrode 723 on a lower surface thereof. Thereafter, solder layers 732 are plated on both electrodes, respectively. On the other hand, as a printed board to mount the LED chip thereon, there is prepared a printed board as shown FIG. 2B where wirings 728 (over two areas) for the respective electrodes are printed on an insulator substrate 729 to form a solder resist film 733 and an adhesive agent 731 is painted thereon. Then, seating the above LED chip on the substrate and heating the solder layers 732 for at first melting and subsequently resolidifying, solders 730 connecting the electrode wirings 728, 728 to the LED electrodes 721 and 722 can be completed, respectively. In this way, by arranging the chip laterally on the board and then fixing it thereon, the light can be radiated from the side surfaces of the chip.
The above-mentioned mounting method presupposes a extracting up of a great deal of light from the side surfaces of the chip. Therefore, it is not practical to apply the above mounting method for a conventional LED using a multi-layer reflection film on a light absorbing substrate.
On the other hand, it is very important to reduce a threshold current in the semiconductor laser to obtain a high efficiency of converting current to light. In the prior art, Japanese Patent Application Laid-Open Hei 2 No. 170486 discloses a semiconductor laser intended to reduce the threshold current, or to improve the current/light converting efficiency by returning spontaneous emission, which emits in an active layer and does not contribute to an oscillation of the laser, to the active layer (photon recycle). FIG. 3 shows a cross-sectional view of this semiconductor laser.
The semiconductor laser is manufactured as follows: at first, by superimposing an n-type Al.sub.0.2 Ga.sub.0.8 As layer of a thickness .lambda./4n(n: refractive index of medium) on an n-type AlAs layer of a thickness .lambda./4n by turns with 10 cycles by means of MOCVD (metal organic chemical vapor deposition) method, an n-type multi-layer reflection film 902 is fabricated on a n-type GaAs substrate 901. Then, after forming a n-type Al.sub.0.3 Ga.sub.0.7 As cladding layer 903, a GaAs active layer 904 and a p-type Al.sub.0.3 Ga.sub.0.7 As cladding layer 905 in order, p-type Al.sub.0.2 Ga.sub.0.8 As layers and p-type AlAs layers are mutually laminated to have .lambda./4n in thickness, respectively, with 10 cycles, whereby a p-type multi-layer reflection film 906 is formed and then a p-type GaAs cap layer 907 is overlaid thereon. Next, after forming a mesa-stripe by etching, a p-type Al.sub.0.6 Ga.sub.0.4 As buried layer 908 and a n-type AlGaAs buried layer 909 are provided by LPE method (Liquid Phase Epitaxy). Then, a Zn diffusion area 910 is formed by selective diffusion and a p-electrode 911 and a n-electrode 912 are provided. Then the semiconductor laser is completed to be of 100 .mu.m in oscillator length by cleavage.
In the above semiconductor laser, however, since the multi-layer reflection film affords high reflectivity for limited light in a specific incident direction (a vertical incident light in case of .lambda./4n in layer thickness), there is risen a problem that, although a beam traveling from the active layer 904 to the multi-layer reflection film 902 or 906 is reflected by the multi-layer reflection film to contribute to the photon recycle, an incident beam oblique to the multi-layer reflection film is so absorbed that it does not contribute to the outward radiation.
Furthermore, since the semiconductor laser needs high accuracy in thickness of each layer of the multi-layer film, it is difficult to manufacture. Again, because of many boundaries between different kinds of semiconductors, the semiconductor laser is apt to have an increased resistance.